To train combat pilots effectively, training flights are conducted over a test range which includes simulated missile defense systems. These simulated missile defense systems include radar beacons which simulate fire control radar of missile defense systems. In order to evaluate a pilots performance during such training, it is necessary to monitor accurately the position of the aircraft as it travels through the test range.
U.S. Pat. No. 5,144,315, issued Sep. 1, 1992, entitled "System for Accurately Monitoring Aircraft Position During Training Exercises" (the '315 patent), discloses a system in which a conventional airborne IFF transponder located on board an aircraft is used as a source of tracking data. Emissions from the airborne IFF transponder are used in an inverse Loran position determining system for establishing longitudinal and latitudinal coordinates for the aircraft. In the inverse Loran positioning system, a plurality of ground receive stations are positioned around a test range. An aircraft flying through the test range includes a squitter transmitter which interrogates a conventional airborne IFF transponder. In response to the interrogation, the transponder transmits its altitude and identity on a continuous basis to ground receive stations in the test range. The ground receive stations have accurate clocks which are synchronized with each other and which are used to note the time of arrival of transmissions received from the airborne IFF transponder.
In the system disclosed in the '315 patent, each of the ground receive stations is linked to a master ground receive station through a wide-band, two-way data link. The two-way data link enables the master ground receive station to calibrate clocks at remote ground receive stations as well as to receive data from the remote ground receive stations identifying time of arrival of transponder transmissions emitted from aircraft passing through the test range. Using well known Loran techniques, the master ground receive station calculates X-Y coordinates for the aircraft using the time of arrival information received from the remote ground receive stations. Additionally, altitude information which is transmitted from the IFF transponders is used to monitor the altitude of the aircraft.
As taught by the '315 patent, an aircraft need only include a squitter transmitter for interrogating a standard IFF transponder on the aircraft. Further, the squitter transmitter may be configured to interrogate the IFF transponder on an asynchronous basis to reduce the possibility of transmission collisions with transmissions from other aircraft transponders in the general vicinity. As further taught by the '315 patent, an accelerometer signal representing aircraft performance (for example, changes in flight path) may be used to modify the squitter transmitter average interrogation rate. The resultant change in average reply rate of the IFF transponder may be used by the ground receive stations and the master ground receive station to position accurately ground tracking stations or illuminators in light of the changes. As still further taught by the '315 patent, it is possible to cross-link a TACAN interrogating system on the aircraft with the IFF transponder. In such embodiments, transmission of a TACAN pulse is used to initiate interrogation of the aircraft IFF transponder. In this case, each of the ground receive stations receives the TACAN pulse as well as the IFF transponder signal containing altitude data. If each aircraft is assigned a TACAN frequency, the altitude and TACAN frequency may be correlated to provide the identity of the aircraft.
The '315 patent teaches the use of two types of interrogation pulses which are spaced at a fixed distance from one another in time. The IFF transponder emits a signal which provides an identification in response to the first type of interrogation pulse and emits a signal which provides an altitude in response to the second type of signal. However, in accordance with standard IFF transponder reply codes, 12 bits are available for use in providing identification information. In certain applications, the use of 12 bits limits the number of aircraft that can be identified.
In light of the above, there is a need in the art for a method and apparatus for monitoring the position of vehicles such as aircraft and/or ground vehicles in the area of a test range which has a greater capacity for identification than is provided by the typical IFF transponder reply code.